Embodiments of the present invention relate generally to liquid-filled power transformers, and, more particularly, to a system for cooling such transformers to minimize externally radiated heat, while providing a smaller footprint and thus more options for deploying such a transformer.
Transformers, and similar devices, come in many different shapes and sizes for many different applications and uses. Fundamentally, all of these devices include at least one primary winding(s) with at least one core path(s) and at least one secondary winding(s) wrapped around the core(s). When a varying current (input) is passed through the primary winding a magnetic field is created which induces a varying magnetic flux in the core. The core is typically a highly magnetically permeable material which provides a path for this magnetic flux to pass through the secondary winding thereby inducing a voltage on the secondary (output) of the device.
Power transformers are employed within power distribution systems in order to transform voltage to a desired level and are sized by the current requirements of their connected load. If a load is connected to the secondary, an electric current will flow in the secondary winding and electrical energy will be transferred from the primary circuit, through the transformer, to the load. Transformers are designated by their power rating, typically in kVA, which describes the amount of energy per second that they can transfer and also by their primary and secondary operating voltages, typically in kV. Medium power transformers can be rated up to 10,000 kVA and up to 46 kV while large power transformers can be rated up to 120,000 kVA and up to 345 kV.
One shortcoming of existing transformers is their susceptibility to operational problems associated with high temperatures of operation, both internal and external to the transformer. The largest source of heat in a transformer is heat created by the load current flowing through windings of the core-winding assembly, based on the inherent resistance of the wire from which the windings are constructed. High temperatures for long periods of time in transformers will destroy insulation positioned about and between the windings, thereby leading to a transformer failure. During the design of power transformers, considerable effort is spent to: reduce losses so as to decrease the generation of heat in the windings; move heat away from the windings (i.e., provide cooling) and spread the heat out by physical design (i.e., provide heat dissipation); and improve the winding insulation so that it can withstand greater exposure to heat.
With regard to providing cooling to the transformer windings and heat dissipation from the transformer, one common solution in to construct the transformer as a liquid-filled transformer. In a typical liquid-filled power transformer, a bath of dielectric insulating liquid is contained within the housing of the transformer, with the core and windings of the transformer being submerged in the dielectric insulating liquid. Moving heat away from the windings is accomplished by direct contact of the windings with the dielectric insulating liquid. The denser the dielectric insulating liquid the better the heat transfer and, as such, the typical liquids used are selected both for their dielectric properties (insulating the high voltage) as well as their heat transfer properties.
In operation of a liquid-filled transformer, it is recognized that as heat is moved away from the windings and transferred to the dielectric fluid, a heat-exchanging mechanism for dissipating heat in the dielectric fluid is required. One existing type of liquid-filled power transformer is shown in FIG. 1, with the transformer 100 including a housing 102 having a dielectric liquid 104 therein that immerses a core 106 and winding 108. The transformer 100 includes external radiators 110 exposed to ambient air, which provide the dielectric insulating liquid 104 a path to circulate through a region of increased surface area for the purpose of liquid-to-air heat exchange to cool the dielectric insulating liquid 104. The radiators 110, through convection, move the hot liquid 104 through a series of channels 112 providing more surface area for the air outside of the housing 102 to contact the radiator 110 to remove heat from the liquid 104. To provide improved cooling, the radiators 110 are often equipped with large fans 114 to provide additional forced-air cooling. To provide further improved cooling, the radiators 110 with, or without, fans 114 are often connected to the housing 102 through large pumps 116 to provide additional forced-oil cooling. However, the addition of radiators 110, associated fan systems 114, and associated pump systems 116 external to the main housing 102 of the transformer 100 comes as a tradeoff in transformer size and cost, and often doubles the footprint of the transformer 100.
Another existing type of liquid-filled power transformer is shown in FIG. 2, with the transformer 120 including a heat-exchanging mechanism in the form of a secondary cooling loop or system 122 that provides for liquid-to-liquid cooling of dielectric insulating liquid 104 diverted out of the transformer 120. Secondary cooling system 122 is in the form of a forced water cooled unit, for example, that includes a cooling unit/heat exchanger 124 that pumps water 126 to/through a radiator unit 128. Radiator unit 128 is positioned with a reservoir 130 which is configured to hold a quantity of dielectric insulating liquid 104 pumped out from housing 102 by way of pumps 116. The dielectric insulating liquid within reservoir 130 is cooled by way of a liquid-to-liquid transfer of heat energy with water 126 of cooling system 122. However, similar to the system of FIG. 1, the addition of reservoir 130, radiator unit 128 and heat exchanger 124 external to the main housing 102 of the transformer 100 comes as a tradeoff in transformer size and cost, and often doubles the footprint of the transformer 100.
Still another existing type of liquid-filled power transformer provides for diverting of dielectric insulating liquid out of the transformer housing to a remote heat exchanger unit. However, cooling of the dielectric insulating liquid in such a manner is limited by the amount of work it adds to the dielectric liquid to move it out of the insulating/dielectric environment and to the heat exchanger. That is, the work (e.g., pumping) done on the dielectric liquid can cause frothing and foaming, and if the dielectric liquid is left in this frothed state upon reentry into the transformer, the dielectric strength and heat transfer capability of the liquid will be severely compromised. Another disadvantage of this type of system is that if the piping between the transformer and the remote heat exchanger were to develop a leak, the amount of insulating liquid in the transformer would decrease to a level that may be insufficient for operation of the transformer, thereby leading to an eventual failure of the transformer. Lastly, this type of increased path for the dielectric insulating liquid leads to an increased likelihood of contaminants being introduced into the liquid, thereby resulting in a contaminated liquid having lower dielectric strength and heat transfer capability.
As a result of the existing transformer configurations set forth above, liquid-filled power transformers have historically been large in size (with respect to their rating) and thus have typically been located outdoors from a facility, such as on a rooftop or on a concrete pad in a fenced-in or controlled area. Such placement of the transformer necessitates running large secondary feeders for long distances at great cost to connect the transformer to its designated load center inside of a building.
Additional advancements in dielectric insulating liquid technology have brought about liquids that are less flammable and have improved dielectric properties. Such fluids have a higher fire point, thereby allowing for placement of the transformer inside of a building. Beneficially, placement of the transformer inside of a building (close to the load) allows for the length of the secondary cables for transferring power from the transformer to the load to be greatly reduced, which results in substantial cost savings in both materials and installation, as well as cost savings with respect to the long-term cost of continuous losses during normal everyday operation. However, drawbacks still exist regarding the placement of existing liquid-filled transformers indoors. For example, the inclusion of radiators and associated fan systems to the transformer still results in a large transformer. Additionally, the placement of existing liquid-filled, and dry-type, air-cooled transformers indoors also results in transferring heat from the normal operation of the transformer to the inside of the building (i.e., the liquid-to-air heat exchange from the radiator to the surrounding ambient environment), which then needs to be removed from the building.
Therefore, it would be desirable to provide a system and method for cooling a transformer that overcomes the disadvantages of known cooling techniques for liquid-filled transformers, especially for deployment indoors. It would further be desirable to provide a transformer cooling system and method to enable deployment of the liquid-filled power transformer in a building without adding heat to the building that would then need to be removed by the building air condition system. It would also be desirable to provide such a system and method that enables the size of the transformer to be reduced to optimize installation in a building.